According to the preamble of the main claim, the invention relates to a steering system for a motor vehicle and to a method for operating the steering system.
A steering system for a motor vehicle with steerable wheels has in many cases, together with a steering organ actuatable by the driver, an actuator for the angle of lock of the steerable wheels. Electrical or hydraulic servo-assisted devices parallel to a mechanical transmission device are used as actuators or as exclusive actuators in so-called xe2x80x9csteer-by-wirexe2x80x9d systems.
Such a xe2x80x9csteer-by-wirexe2x80x9d system usually comprises also an electronic control device and has been disclosed, for ex., in DE-OS 196 32 251.
Most modern vehicles are undermodulatedly designed in the curve limit range. When approaching the curve limit range there first occurs on the steered front wheels a transversal slip so that the vehicle moves and decelerates via the front wheels. One other steering angle leads only to too reinforced deceleration but with diminishing transversal acceleration. An experienced driver recognizes the undermodulation and steers only as far as the existing driving situation allows.
However, many inexperienced drivers react erroneously in such a situation. Wishing to evade an obstacle, for example, they continue to steer further with the result that even though the vehicle decelerates, it does not follow the desired curve radius. This mistaken reaction is specially critical in case of changing road condition: if fully steering in the way described, for ex., on even road and the front wheels finding a non-skidding road section suddenly stop again, the vehicle can run out of control.
The problem on which the invention is based is to provide a steering system and a steering method which assist even inexperienced drivers in avoiding the mistaken reaction described.
The problem is solved by a steering system having the features of the main claim and a steering method for operating the steering system according to the first method claim. Features of the sub-claims give advantageous developments.
According to the invention a device is provided for determining a transversal slip occurring on the steerable wheels. In the electronic control device can be determined an upper threshold value for the transversal slip which is compared with the value of the transversal slip determined. There is provided at least one means which serves to counteract a further enlargement of the angle of lock when the transversal slip determined is greater than the upper threshold value. As standard for the occurring transversal slip can be used, for example, the oblique transit angle. In the electronic control device this can be calculated from the signal, anyway existing in vehicles having electronic driving dynamics, of a yawing rate sensor, of wheel position sensors and of the vehicle speed. The threshold value for the transversal slip is in the simplest case a presettable fixed value stored in the electronic control device. But it is also possible to determine dynamically the threshold value according to driving state parameters like speed. If the actually occurring transversal slip or the oblique transit angle is larger than the upper threshold value, it is concluded that the driver has mistakenly reacted by steering too far. For this case at least one means is provided which serves to counteract a further enlargement of the angle of lock. It is convenient to this case that the driver be assisted in avoiding his mistaken reaction whereby the driving safety is increased.
Too large an angle of lock can reliably be prevented if the means which counteracts a further enlargement of the angle of lock is the actuator of the angle of lock, the electronic control device upwardly limiting the regulating distance of the actuator.
A less strong engagement in the steering function results when there is provided a warning signal generator controlled by the electronic control device and that generates a warning signal perceivable by the driver when the transversal slip determined is larger than the upper threshold value.
In steering systems without mechanical connection between steering organ and actuator, there is usually provided a steering actuator (wheel) which generates the steering force perceivable by the driver. It is convenient to use the steering actuator (wheel) as warning signal generator. Hereby is made possible a steering method in which the steering organ actuator produces an increased steering force when the transversal slip detected is greater than the upper threshold value. The driver senses a very clear feedback when the steering force generated increases suddenly by a presettable amount upon exceeding the upper threshold value. Alternatively or additionally to increased steering force the steering organ actuator can produce a vibration of the steering organ. Similarly to an ABS deceleration where the brake pedal vibrates, a vibrating steering wheel makes the driver aware that he is in a limit range of tire adhesiveness. The different warning signals can obviously be combined with each other or other warning signal generators which operate acoustically or optically can also be used.